1. Field of the Invention
The present invention relates generally to battery chargers and more particularly to a battery charger that receives power from a limited power source, such as a Universal Serial Bus (USB) power interface.
2. Description of the Related Art
Many electronic devices operate with a low direct current (DC) voltage supply such as, for example, 5 volts DC. The DC voltage supply may be derived from an alternating current (AC) adapter (e.g., by transforming, rectification and filtering of an AC voltage), or it may be supplied by one or more internal batteries of the electronic devices. Often, such a device obtains DC power from the AC adapter when the AC adapter is connected to the device and to an AC source and obtains power from the batteries when the AC adapter is not supplying power. If the batteries are rechargeable batteries, the device advantageously provides a portion of the DC power from the AC adapter to the batteries via a charging current to charge or to maintain the charge of the batteries.
Devices are presently available that are connectable to a USB interface of a computer system via a USB cable. Although a USB device may have an independent AC adapter to provide DC power to the device, a low power device may obtain power from the computer system via a conductor in the USB cable. If the USB device has internal rechargeable batteries, a portion of the DC power may be used by a charging circuit to charge or to maintain the charge of the batteries. Generally, the DC power available via the USB cable is current limited. Thus, if the batteries are being charged while the device is operating, sufficient current may not be available to sustain the proper operation of the device unless the current supplied to charge the batteries is limited. Thus, the charging current to the batteries is usually fixed and limited to the excess current available when the USB device is utilizing a maximum current. On the other hand, if the charging current to the batteries is always limited to the excess amount of current available when the device is operating at its maximum power, the amount of time required to charge the batteries may be excessive.